This invention relates to multiplexing circuitry which can handle very high densities of high impedance inputs.
The particular problems addressed by this invention have been encountered in handling the analog signals from arrays of very small infrared detectors located on a focal plane. The manufacture of such focal plane detector arrays is discussed in various patents and applications assigned to the assignee of this application, including U.S. Pat. Nos. 4,290,844 (issued Sept. 22, 1981), and 4,304,624 (issued Dec. 8, 1981), and application Ser. No. 282,459 (filed July 13, 1981).
In order to convey each detector signal individually to the process electronics, multiplexing circuit adjacent to the focal plane is necessary. Such multiplexing circuitry should be (a) able to handle high impedance inputs, (b) low in noise, (c) free of substantial cross-talk, (d) capable of very low temperature operation, (e) capable of very quick settling time, and (f) confined to the dimensional limitations of integrated circuits.
While the present invention was motivated by the needs in the field of high-density infrared detector arrays, its principles may be applied in other fields where similar problems occur.
Because of the low temperature environment needed by infrared detectors, the electronics used at the focal plane are limited to those elements capable of operating under such low temperature conditions, e.g., 30.degree. Kelvin-150.degree. Kelvin.
Heretofore, the primary efforts to solve the problems addressed by this invention have involved the use of charge coupled devices (CCDs). Multiplexers using CCDs have, however, encountered serious difficulties. In experimental applications, such multiplexers have suffered from marginal noise performance, and their dynamic range is relatively low for the needs addressed in the present disclosure.
Other multiplexer circuit designs (not using CCDs) have been tried, which have had high power dissipation because the signals awaiting their turn in the multiplexer have constituted a power drain.
Another problem in multiplexers of the type under consideration has been excessive cross-talk among the varying voltages which exist on different paths in extremely confined space.
The present invention is intended to solve the problems outlined above, and generally to provide a multiplexer which has greatly improved overall performance, together with a potential reduction in cost.